a) Field of the Invention
The present invention relates to a transmission system for synchrotron radiation light (SR light), and more particularly to an SR light transmission system capable of giving an intensity distribution to SR light in a cross sectional plane perpendicular to its optical axis and to an SR light transmission system for irradiating a certain area by swinging up and down a transporting direction of the SR light using a swinging mirror.
b) Description of the Related Art
With reference to FIG. 1, the structure of a conventional X-ray exposure system will be described. Reference to FIG. 1 is also made when embodiments of the invention are described later.
The X-ray exposure system comprises of an SR light generator unit 1, an SR light transmission unit 10 and an X-ray stepper 50.
The SR light generator unit 1 comprises of a vacuum room 2 and an electron beam circular orbit 3 formed therein. SR light is radiated from electrons moving along the circular orbit 3. This SR light is output from a beam output port of the vacuum room 2.
The SR light transmission unit 10 has an incoming vacuum duct 11, a mirror box 12 and an outgoing vacuum duct 30. Incoming opening 13 and outgoing opening 14 are formed in the wall of the mirror box 12. The incoming vacuum duct 11 hermetically communicates the beam output port of the SR light generator unit 1 with the incoming opening 13. In the vacuum duct 11, a vacuum shielding valve (not shown), an SR light shielding shutter (not shown) and the like are mounted. The input port of the outgoing vacuum duct 30 is hermetically coupled to the outgoing opening 14.
A reflection mirror 15 is disposed in the mirror box 12 and supported by a mirror swinging mechanism 16. SR light entering the mirror box 12 via the incoming opening 13 is reflected by the mirror 15 and enters the outgoing vacuum duct 30 via the outgoing opening 14. The mirror 15 is disposed such that its incidence plane contains an optical center axis of incidence SR light and a normal to a reflection plane at the reflection point and such that an angle between the optical center axis and the reflection plane is about 1 to 2.degree., i.e., such that the incidence angle is about 89 to 88.degree..
The swinging mechanism 16 swings the mirror 15 along an axis vertical to the incidence plane and passing the reflection point of SR light, i.e., a horizontal rotary shaft is used as a swing axis. As the mirror 15 swings, reflected SR light is swung up and down. The swing axis may be set to a position different from the reflection point of SR light.
An output window made of a beryllium thin film is formed in a window flange 37 which is hermetically mounted on an output end of the outgoing vacuum duct 30. SR light entering the outgoing vacuum duct 30 transmits through the output window formed in the window flange 37 and is radiated to the outside of the vacuum duct 30. An X-ray stepper 50 is disposed facing the window flange 37. The X-ray stepper 50 holds a semiconductor substrate 51 at the position where SR light radiated from the window flange 37 is applied. An exposure mask 52 is supported in front of the semiconductor substrate 51.
Although SR light is irradiated omnidirectionally in the horizontal plane, it only has a spread of about +/-1 mrad (mili-radian) in the vertical plane. By swinging the mirror 15, SR light is swung in the vertical direction so that SR light can be applied to a broad surface area of the semiconductor substrate 51.
SR light diverges in the horizontal direction. This SR light is therefore converged in the horizontal direction to make it parallel light fluxes, so that SR light radiated from the light source can be more efficiency used. If the intensity of X-ray is increased, the X-ray exposure time can be shortened.
In order to converge SR light in the horizontal direction, as the mirror 15 shown in FIG. 1, a cylindrical mirror or a toroidal mirror is used. A substantial focal length of a cylindrical mirror or toroidal mirror changes with an incidence angle of SR light. As the mirror 15 is swung, the incident angle changes and the focal length with respect to the horizontal plane changes with the incidence angle correspondingly.
As the focal length changes, an energy density of SR light on the surface of the semiconductor substrate 51 changes. It is therefore difficult to uniformly apply X-rays to the surface of the semiconductor substrate 51.
SR light reflected by a cylindrical mirror or a toroidal mirror has a shape extending along generally a circular line in the cross sectional plane (beam cross section) perpendicular to the optical axis. Therefore, an SR light radiation area on the exposure surface of the semiconductor substrate 51 also has a shape extending along generally a circular line. SR light can be applied to a broad area by moving this radiation area in the radial direction passing through a center point of the circular line.
A length of the circular radiation area cut along a straight line parallel to the motion direction of the area becomes longer at a position more remote from the center of the radiation area in the horizontal direction. In addition, the exposure amount on the exposure plane obtained by swinging the mirror and moving such an exposure area in the vertical direction on the exposure plane becomes larger at a position more remote from the center of the exposed area. It is therefore difficult to uniformly expose the exposure plane by using SR light having a circular radiation area.